Polymeric skins for trim products, especially motor vehicle trim products, are currently made with rotocasting or slush molding techniques. Common motor vehicle trim products include instrument panels, door trim panels, armrests, headrests, seat covers, and similar products. Other industries in which polymeric skins may be used include the aviation and marine industries.
Rotocasting and slush molding both use heated molds to melt a powdered casting material such as polyvinyl chloride (PVC). In the rotocast process, molds are inserted into large ovens in which the air temperature is far above the melting point of the casting material, for example about 100.degree. C. (180.degree. F.) to about 400.degree. C. (720.degree. F.) above the melting point of the casting material. When the mold interior surface reaches a temperature slightly higher than the melting point of the casting material, the casting material is dumped into the mold. The mold is then rotated to distribute the casting material over the surface of the mold with a combination of gravity and mold movement. Slush molding is a variation on rotocasting in which the mold cavity is filled with casting material. Only the casting material near the hot mold wall melts. Excess casting material is dumped out of the mold. In both rotocasting and slush molding, the casting material often is distributed uniformly (or uniformly enough) over the mold surface. Neither method, however, allows precise control over the distribution of the casting material. As a result, skins made by rotocasting and slush molding can have undesired nonuniformities that lead to defective products. Because these methods lack a means for precisely controlling the distribution of the casting material within the mold, they are unsuitable for making multicolor skins or practicing other molding techniques that require precise control over casting material distribution.
Rotocasting and slush molding methods also suffer from other drawbacks. For example, the casting equipment, especially the oven, is very large. As a result, the equipment requires large amounts of factory floor space. Second, both methods consume large amounts of energy because of the high heat losses associated with moving molds into and out of the oven. In addition, heating the molds to high temperatures can char or burn the skins, resulting in unsatisfactory products that must be reworked or scrapped.
Therefore, what is needed in the industry is a method of molding polymeric skins that can make multicolor skins and permits precise control over casting material distribution.